1. Field of the Invention
The present invention relates to an image correction method for correcting a nonejection state, which is an inherent characteristic of each recording head of an inkjet recording system that ejects ink dots onto a recording medium to form an image thereon.
2. Description of the Related Art
Along with the popularization of copying machines, information processing equipment such as word processors and computers, and communication equipment, digital image-recording apparatus using inkjet recording heads have come into widespread use as image-forming (recording) apparatuses for the aforesaid equipment. Also, recent enhancements in image quality and colorization of visual information in the information processing equipment and communication equipment has necessitated concomitant enhancements in image quality and colorization in recording apparatuses.
In such a recording apparatus, for miniaturizing and speeding up the forming of a pixel, a plural-recording-elements integrated recording head (also referred to as a multi-head) is used, in which plural ink nozzles and ink paths are integrated in high density. Furthermore, for colorization, the apparatus generally has plural multi-heads corresponding to respective colors of cyan, magenta, yellow, and black. Using this structure, technology has strived to output high grade images at high speed and at low cost. In one method to increase speed, a one-pass high-speed method, in which the length of the multi-head is about the width of a recording medium, is coming into use.
For example, in transverse-feed page printers for A-4 size paper, the length of the multi-head is about 30 cm, and 7000 nozzles or more are required to achieve 600 dpi images. It is extremely difficult to manufacture such multi-heads having such a large number of nozzles without some defects. In addition, the nozzles will not necessarily have the same performance characteristics. Furthermore, some nozzles become incapable of ejection after being used. Therefore, it is worth noting head shading techniques for correcting density nonuniformity due to ejection-amount nonuniformity and deviations in landing position (kink), as well as nonejecting-nozzle correction (nonejection complementary) techniques for performing complementary processing on a nonejecting nozzle to enable even a multi-head with defects to be used.
Generally in head shading techniques, the density is measured for every nozzle and the input-image data is then corrected for the measured result. For example, if the ejection amount of one nozzle is reduced for some reason so as to reduce the density corresponding to that nozzle, this technique corrects the input image data so that a gradation value corresponding to the affected nozzle is increased so as to yield uniform density throughout the printed images.
The nonejection complementary technique, described in another U.S. patent application, (U.S. Ser. No. 845,498) assigned to the same assignee as this application, sets forth other methods for collecting nozzle output variations. If one nozzle for cyan is nonejecting, for example, methods for compensating for this ink shortage include (i) substituting with the ejection of nozzles on both sides of the nonejecting nozzle (adjacent complementation), (ii) complementing the nonejecting nozzle with an ink dot of another color, such as black, (different-color complementing), and (iii) distributing the data corresponding to the nonejecting nozzle to nozzles at both ends of the head.
The above-mentioned patent application is especially effective in a recording apparatus using a full-line head, which corresponds to those heads that span the entire width of the recording sheet.
With respect to the different-color complementing described above, a method has been proposed for determining the amount of the different-color ink to be complementarily ejected, which uses pixel-image density data (a gradation value) determined as a function of the number of successive nonejecting nozzles.
However, the different color complemented result often may vary from that anticipated, depending on the ejection condition of the adjacent nozzles. For example, when the amount of the ink ejected from the adjacent nozzles on both sides is large so as to increase the size of an ink dot, if the amount of different-color complementing ink is not reduced from the determined standard amount (hereinafter the amount of the complementing is referred to as a “reference different-color complementing amount”), the resultant complementing may become conspicuous due to the effect of the large number of ink dots adjacent to the nonejecting nozzle. That is, it is necessary to determine the amount of the different-color complementing by measuring the degree of the effect on the vicinity. This situation is shown in FIG. 1.
Solid lines in FIG. 1 show density changes when a zigzag pattern having a duty factor of 50% (a checker pattern, in which dots are recorded at a percentage of 50%) is formed with ink dots of about 60 μm at a resolution of 600 dpi. In the drawing, symbols (A1) to (A3) show the case that the dot diameter from the nozzles on both sides of the nonejecting nozzle is the same as that from other nozzles, and the number of successive nonejecting nozzles for each case is 1, 2, and 3, respectively. Symbols (B) and (D) show cases where the dot diameter from the nozzles on both sides are smaller by 4 μm and 7 μm, respectively. Symbols (C) and (E) show cases where the dot diameter from the nozzles on both sides are larger by 4 μm and 7 μm, respectively. In such a manner, it is understood that the density in the vicinity of the nonejecting nozzle is changed by the ink ejection characteristics of the nozzles on both sides.
When the ejection by the nozzles on both sides of the nonejecting nozzle is the same in dot diameter and dot density as that in the other nozzles, and only the landing position of the ejection is shifted in the nozzle-line direction (Y kink), the appearance is slightly different from the above-mentioned case in which the dot diameter is changed. Solid lines in FIG. 2 show density changes when the Y kink of the nozzles on both sides of the nonejecting nozzle is different, and similarly to FIG. 1, FIG. 2 shows a zigzag pattern having a duty factor of 50% and which is formed with ink dots of about 60 μm at a resolution of 600 dpi. In the drawing, symbols (A1) to (A3) show cases where there is no landing-position shift (Y kink) in the nozzles on both sides of the nonejecting nozzle. Symbols (B) and (D) show cases where the landing position of the nozzles on both sides are shifted by 7 μm and 14 μm in the direction opposite to the nonejecting nozzle, respectively. Symbols (C) and (E) show cases where the landing position of the nozzles on both sides are shifted by 7 μm and 14 μm, in the direction toward the nonejecting nozzle, respectively. Similar to the above-mentioned case, in which the dot diameter is different, the density in the nonejecting nozzle changes depending on conditions of the nozzles on both sides. However, when about five pixels are viewed in the vicinity of the nonejecting nozzle and including that nozzle, the respective amounts of ink are substantially the same, and only changes in the density corresponding to the nonejecting nozzle are apparent. Therefore, if the ejection by the nozzles on both sides of the nonejecting nozzle is the same in dot diameter and dot density as that by the other nozzles, and only the landing position of the ejection is shifted, the standard different-color complementary amount can substantially have the same advantages.
From these factors, the ejecting conditions of nozzles in the vicinity of the nonejecting nozzle, specifically dot density, dot diameter, and kink, can be comprehended, and then, if there are no fluctuations in the dot density and dot diameter, the complementing may be performed with the reference different-color complementing amount. However, if there are fluctuations in the dot density and dot diameter, the complementing must be performed with an amount increased or decreased from the reference different-color complementing amount by referring to the density of the nonejecting nozzle portion.
However, typical reading devices (scanner) scarcely read dot density and existence of an ink dot of approximately 60 μm; and as for the kink, although a kind of smaller kinks approximately several dozen μm can be recognized, especially those of several μm, cannot be recognized by the scanner.
It is not cost-effective to perform the correction with a high-efficiency scanner capable of reading the density, size, and position of an ink dot of several μm.